1. Field of the Invention
The present invention relates to systems for detecting the positions of benchmarks on a recording medium. It particularly relates to systems where the direction and magnitude of the displacement of a transducer from being centrally disposed over a servo track recorded on a moving medium is indicated by the output of a decoder in receipt of signals recovered from the servo track by the transducer. Most particularly it relates to such systems where the medium is magnetic tape or disc for the recording of informational data signals.
2. The Prior Art
It is well known in the field of magnetic data recording on moving tape or rotating disc, and latterly in the field of video recording and optical data recording on disc, to employ one or more servo tracks on the tape or disc to act as benchmarks to that signal recovering or recording transducers can be located over the medium in positions measured relatively to the servo track to accurately locate other data signals. It is usual to employ a decoder in receipt of signals recovered from the medium by the transducer, the decoder providing an output signal indicative of the sense and magnitude of displacement of the servo-signal recovering transducer from being centrally disposed over the servo track. The output of the decoder can then be variously used to give indication of the postion of the servo track relatively to an external position measuring system to a controller for the controller to use that information to locate the transducer over other, data storage tracks by interpolation or extrapolation, or directly as a position feedback signal in a transducer-positioning servo mechanism to center the transducer over the servo track.
There are many different types of servo tracks sharing the common factor of consisting in two associated sub-tracks. Signal attributes from the sub-tracks are picked up in common by the transducer, separated and compared to indicate the position of the transducer relative to central disposition over the track, central disposition being indicated by equality of the attributes. The so-called dibit system and the so-called tribit system are characteristic of this class. In both cases the decoder measures the amplitudes of position-indicating pulses and subtracts one measurement from the other to provide an output indication by magnitude and sense of the size and direction of the displacement of the transducer from being centrally disposed over a median line between two contiguous sub-tracks. The pulse amplitude measuring process and the amplitude comparison process are both subject to errors due to the relatively large numbers of components required for their execution by the decoder and the statistical variation in component values.
In both the so-called dibit system and so-called tribit system the linear density of servo signals must be kept low so that successive pulses in the dibit or tribit patterns as recovered by the transducer are far apart from one another compared to the widths of the pulses, so that the pulses do not unduly interfere with one another to disturb their relative values.
It is the trend that an increasing number of data storage tracks are required in closer proximity to one another on the medium, be it disc or tape, magnetic or optical, and in such high track density situations, the output errors produced by a conventional decoder as a result of its multiplicity of components and operations can render the subsequent positioning of a transducer insufficiently accurate for the proper recording and/or recovery of signals.
It is also the trend that servo information is coming to be recorded interspersed linearly among data signals on common tracks, the decoder being made operational only when servo signals are being recovered by the transducer, thus allowing the transducer to be precisely positioned over the common tracks. In such situations it is deleterious to the data storage capacity of the common tracks to employ a servo system wherein position-indicating pulses must be well separated from one another, as this eats into the part of the common tracks potentially usable for informational data.
Conventional servo signal decoders, because of the high component count and varied operations are required to employ high precision individual components and/or components adjustable during manufacture in order to reduce the many possible sources of error. This causes the conventional servo signal decoder to be high in cost.
It is therefore desirable to provide a servo track position detection system for detecting the position of a transducer relatively to central disposition over a servo track from which it is recovering signals wherein the individual pulses of the servo signal can be recorded in close proximity to one another and wherein the decoder is of simple construction with a low component count and relatively few operations.